This project examines behavioral and physiological development in rhesus monkeys, focusing on individual differences in temperament reactivity and their developmental consequences in different rearing environments. Research this past year focused on 4 studies. In the first study, adolescent monkeys with previous separation experience as infants and juveniles were removed from their social groups for four 4-day periods, and behavioral and physiological measures were obtained prior to, during, and after each separation. Analyses of behavioral and neurohormonal data revealed that (a) individual differences in stress reactivity remained quite stable from infancy through adolescence even in the face of major developmental changes in behavior and physiology, (b) these diffrences were masked during periods of stable group living, and (c) although behavioral reactions to separation changed dramatically as monkey entered puberty, physiological patterns of arousal remained the same. A second study using these same monkeys examined the effects of the antidepressant imipramine prior to, during, and following subsequent brief separations. Preliminary results showed that the effects of imipramine were different in monkeys who displayed extreme reactions to previous separations than in monkeys whose reactions to previous separations were mild. The third study developed a neonatal assessment system designed to identify individual differences in reflex and temperament development in rhesus monkey neonates and to use the assessments to predict subsequent individual differences in stress reactivity. Results to date indicate that the assessment system is sensitive to early rearing condition differences among subjects, that temperament and muscle tone measures can identify neonates at risk for high stress reactivity later in life, while certain orienting measures seem to predict subsequent hyperactivity. The fourth study, initiated this past year, uses these neonatal measures to identify potential high and low reactive infants, who are then cross-fostered with multiparous females who differ dramatically in their style of mothering.